tools: Allow optional arguments after file arguments in test_backend.

[shubham-padia]

Fixes #9233.
Uses nargs='' instead of nargs='argparse.REMAINDER'.
nargs='argparse.REMAINDER' gathers remaining terms as arguments
even if it is an option e.g --coverage, while '' gathers all the
command-line arguments until the next option is encountered.

From https://docs.python.org/3/library/argparse.html#nargs:

• '*'. All command-line arguments present are gathered into a list.:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', nargs='*')
>>> parser.add_argument('--bar', nargs='*')
>>> parser.add_argument('baz', nargs='*')
>>> parser.parse_args('a b --foo x y --bar 1 2'.split())
Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])

• argparse.REMAINDER. All the remaining command-line arguments are gathered into a list. This is commonly useful for command line utilities that dispatch to other command line utilities:

>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo')
>>> parser.add_argument('command')
>>> parser.add_argument('args', nargs=argparse.REMAINDER)
>>> print(parser.parse_args('--foo B cmd --arg1 XX ZZ'.split()))
Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')

[zulipbot]

Hello @zulip/server-testing members, this pull request was labeled with the "area: testing-infrastructure" label, so you may want to check it out!

[timabbott]

Nice, merged, thanks @shubham-padia!